I have a 2004 chevy silverado with qaud-ra-steer. When i am in 4wd or auto 4wd and i go below 20MPH and turn there is a low grinding or rubbing noise. twice it has kicked it out of 4wd and quadrasteer back into 2wd and 2w steer. This noise is comming out of the front end. this noise only happens when it is in 4wd or auto 4wd. Any ideas what might be going on?

Thanks
Matt